1. Field of the Invention
The present invention relates to a fringe field switching mode liquid crystal display, and more particularly a fringe field switching mode liquid crystal display realizing a high quality screen and reducing production cost at the same time.
2. Description of the Related Art
An In-Plane Switching (IPS) mode LCD has been proposed in order to improve a narrow viewing angle of TN mode LCD. This IPS mode LCD has an improved viewing angle by a structure that a counter electrode and a pixel electrode for driving liquid crystal are disposed on an array substrate and only a major axis is to be seen regardless of the viewing direction.
As described above, the IPS mode LCD has an improved viewing angle, however, it has a low aperture ratio and transmittance since the counter and pixel electrodes are formed of opaque metals. Therefore, a fringe field switching mode LCD (hereinafter referred as FFS mode LCD) has been pro posed in order to improve the aperture ratio and transmittance of IPS mode LCD. The FFS mode LCD is characterized by that liquid crystals are driven by a fringe field.
In the FFS mode LCD, a counter and a pixel electrodes are formed of transparent materials and the distance thereof is narrower than that of substrates. The width is sufficient to drive liquid crystals disposed on the upper part of electrodes.
Accordingly, the FFS mode LCD has an improved aperture ratio by forming the counter and the pixel electrodes with transparent materials and has an improved transmittance by transmitting light at the electrodes.
FIG. 1 is a cross-sectional view illustrating a conventional FFS mode LCD. As shown in FIG. 1, a lower substrate (1) is disposed opposite to an upper substrate (11) at a predetermined distance (d: hereinafter referred as cell gap). A liquid crystal layer (20) comprising a plurality of liquid crystal molecules (20a) is then interposed between the substrates (1, 11). The liquid crystal molecules (20a) have negative or positive dielectric anisotropy.
A counter electrode (2) and a pixel electrode (4) for driving the liquid crystal molecules (20a) are formed on the inner surface of the lower substrate (1) with a gate insulating layer (3) interposed between them. As described above, the counter electrode (2) and pixel electrode (4) are formed of transparent conductors and the distance thereof is narrower than cell gap (d). The counter electrode (2) has a plate shape and the pixel electrode (4) has a silt shape. Although it is not shown, a gate bus line and a data bus line are cross arranged on the inner surface of the lower substrate (1) to define a unit pixel and a thin film transistor is disposed at the intersection of the lines.
A black matrix (12) is then formed on the inner surface of the upper substrate (11) and subsequently, a color filter (13) of red, green and blue (hereinafter referred as R, G, B) is formed between the black matrices (12), that is, on a pixel region.
An over coating layer (14) is formed on the black matrix (12) and color filter (13) to protect the color filter (13) and obtain planarization. The layer (14) comprises a high molecular substance such as acryl and epoxy.
Alignment layers (5, 15) for low pretilt below 3xc2x0 C. are coated on the resultant lower and upper substrate (1, 11) respectively and the alignment layers (5, 15) are rubbed so that rubbing axes thereof may be in anti-parallel with each other.
Although it is not shown, lower and upper polarizing plates are adhered to the outer surfaces of the lower and upper substrates (1, 11). The lower polarizing plate has a polarizing axis in parallel with a rubbing axis of the lower alignment layer (5) and the upper polarizing plate has a polarizing axis perpendicular to that of the lower polarizing plate.
In the conventional FFS-LCD having the above structure, when the counter electrode (2) and pixel electrode (4) have different voltage, the distance between the electrodes becomes narrower than cell gap (d), thereby generating a fringe field. The fringe field has an effect on the liquid crystal molecules on the upper part of the electrodes (2, 4) due to the narrow distance between the electrodes.
Therefore, FFS mode LCD has improved aperture ratio and transmittance when compared to IPS mode LCD since almost all liquid crystal molecules in the pixel are driven.
FIG. 2 is a graph simulating transmittance according to the voltage change in a conventional FFS mode LCD. The graph shows the results when the liquid crystal molecules has a dielectric anisotropy (xcex94)xcex5 of xe2x88x924, refractive anisotropy (xcex94n) of 0.077 and phase retardation (xcex94nxc2x7d) of 0.3 xcexcm.
Referring to the graph, when the response speed is 100.00 ms, the transmittance is approximately 45.6%.
However, the FFS mode LCD has a disadvantage of high production cost due to an over coating layer (14) formed on the upper substrate. And, the FFS mode LCD has a difficulty in realizing a high quality screen since a counter electrode and a pixel electrode are all disposed on a lower substrate, therefore it is difficult to eliminating electrostatic and afterimage.
As shown in FIG. 1, a electrostatic protection layer (16) comprising ITO of a thickness below 500 xc3x85 may be formed on the outer surface of upper substrate (11), however, the production cost is increased by formation process of the layer (16).
Therefore, an object of the present invention is to provide a FFS mode LCD of high quality screen by eliminating electrostatic and afterimage.
And, another object of the invention is to reduce the production cost of the FFS mode LCD.
In order to achieve the above objects, the FFS mode LCD according to the present invention comprises: a lower and an upper substrates disposed opposite to each other at a predetermined distance, having transparency; a counter electrode and a pixel electrode disposed on the inner surface of the lower substrate with a gate insulating layer interposed and made of transparent conductors, forming a fringe field when a electric field is applied; a black matrix and a color filter formed on the inner surface of the upper substrate; an ITO layer disposed on the inner surface of the upper substrate including the black matrix and color filter; and a liquid crystal layer interposed between the lower and upper substrates, including a plurality of liquid crystal molecules of negative dielectric anisotropy.
The FFS mode LCD of the present invention further comprises: a first horizontal alignment layer disposed on the top of the lower substrate and having a predetermined rubbing axis; a second horizontal alignment layer disposed on the ITO layer of the upper substrate and having a rubbing axis in anti-parallel with that of the first horizontal alignment layer; a lower polarizing plate disposed on the outer surface of the lower substrate and having a polarizing axis in parallel with the rubbing axis of the first horizontal alignment layer; and an upper polarizing plate disposed on the outer surface of the upper substrate and having a polarizing axis perpendicular to that of the lower polarizing plate.
In the FFS mode LCD according to the present invention, the counter electrode has a plate shape and the pixel electrode has a slit shape. And, the counter electrode has one or more grooves having a width smaller than that of the slit of the pixel electrode, preferably, 2 or 3 xcexcm and the ITO layer includes a window region.